A mixing device for a gas burner with a housing and a Venturi device which is positioned in the housing is known from DE 296 17 621 U1. The housing of the mixing device which is disclosed there has a combustion air inlet, a gas inlet and an outlet for the mixture of gas and combustion air. The Venturi device which is positioned in the housing is constructed as a Venturi nozzle and serves for the mixing of the combustion air and the gas.
A Venturi device, designed as a Venturi nozzle, for a mixing device of a gas burner, which is positioned in a housing of a mixing device and formed from an inlet funnel and a discharge funnel, is known from DE 197 43 464 C1, wherein the Venturi device, specifically the inlet funnel and the discharge funnel of the Venturi device, is contoured to form a contraction section, a mixing section and a diffuser section, and in such a way that the combustion air, issuing from the combustion air inlet, can be fed to the mixing section via the contraction section which narrows in the direction of the mixing section, and that the mixture of gas and combustion air, issuing from the mixing section, can be fed to the outlet via the widening diffuser section. In the mixing section, an inlet-side section of the discharge funnel overlaps a discharge-side section of the inlet funnel, forming an annular gap or an annular gap-like nozzle, via which the gas can be admixed with the combustion air.
For further clarification of the mixing devices of a gas burner, which are known from the prior art, reference may be made to FIG. 1 which shows a schematized cross section of a mixing device 10′ which is known from the prior art. The mixing device 10′ according to FIG. 1 comprises a housing 11′, inside which a Venturi device 12′ is positioned. The housing 11′ of the mixing device 10′ has a combustion air inlet 13′ for combustion air V, a gas inlet 14′ for gas G and also an outlet 15′ for the mixture M of gas and combustion air. The Venturi device 12′, which is positioned in the housing 11′, also has the combustion air inlet 13′ and also the outlet 15′ for the mixture M of gas and combustion air, wherein the gas G, which enters the housing 11′ via the gas inlet 14′, flows radially outward around the Venturi device 12′ and, via an annular gap 16′ which is formed between an inlet funnel 17′ and a discharge funnel 18′ of the Venturi device 12′, can be admixed with or added to the combustion air V. The Venturi device 12′, which includes the inlet funnel 17′ and the discharge funnel 18′, forming a contraction section 19′, a mixing section 20′ and a diffuser section 21′, is contoured in such a way that the combustion air V, issuing from the combustion air inlet 13′ of the housing 10′ or of the Venturi device 12′, can be fed to the mixing section 20′ via the contraction section 19′ which narrows in the direction of the mixing section 20′. The mixture M of gas and combustion air, issuing from the mixing section 20′, can be fed to the outlet 15′ via the diffuser section 21′, which widens in the direction of the outlet 15′. The annular gap 16′ is located in the mixing section 20′. The annular gap 16′ has a cross section which results from a diameter difference between the outside diameter of a discharge-side end of the inlet funnel 17′ and the inside diameter of an inlet-side end of the discharge funnel 18′ in the mixing section 20′. The outside diameter of the annular gap 16′ thus corresponds to the inside diameter of the inlet-side end of the discharge funnel 18′ and therefore to the diameter of the mixing section 20′.
A fan or a blower is typically associated with the outlet 15′ or with the combustion air inlet 13′ of such a mixing device of a gas burner, wherein a speed of the fan or of the blower determines the quantity or the volumetric flow of the mixture of gas and combustion air which is fed to the gas burner. The ratio of gas and combustion air in the combustion air/gas mixture is meant to be relatively constant and is primarily determined by a ratio of the cross section of the inlet funnel 17′ in the mixing section 20′ of the Venturi device 12′ and the cross section of the annular gap 17′ which is formed between the inlet funnel 17′ and the discharge funnel 18′ of the Venturi device 12′. Regardless of the speed of the fan, the ratio of gas and combustion air in the combustion air/gas mixture is supposed to be constant, wherein with increasingly or reducing speed of the fan, undesirable deviations in the ratio of gas and combustion air develop on account of inaccuracies with respect to control engineering. In the case of known mixing devices, the speed of a fan which interacts with the mixing device can be reduced only to a certain extent while maintaining the desired ratio of gas and combustion air. This ultimately results in a realizable modulation range of between 1 and 5, in which a high modulation quality, specifically a desired ratio of gas and combustion air in the combustion air/gas mixture, can be provided. A modulation of 1 corresponds to a full load speed of the fan and a modulation of 5 corresponds to 20% of the full load speed of the fan. With mixing devices which are known from the prior art, the speed of the fan which interacts with the mixing device can therefore be reduced only to 20% of the full load speed of the fan while maintaining the desired ratio of gas and combustion air.